This invention relates to fiber composites and, more particularly, comprises a non-carbon fiber composite over-wrap for an underlying carbon fiber composite member, together with the method for applying the over-wrap to the member.
Due to their relatively high strength-to-weight ratio, carbon fiber composites are the material of choice to fabricate tanks and tubes to contain and transfer cryogenic fluids, e.g., liquid helium, hydrogen and methane. Accordingly, such composites are of particular benefit in fabricating liquid-fuel booster rockets.
A carbon fiber composite is comprised of a carbon fibers embedded in a resin. A problem inherent to the use of such composites in cryogenic applications arises from the difference in the respective coefficients of thermal expansion between the fiber and the resin.
Linear expansion of a rod due to heating is approximated by the equation:L=L0(1+αΔT)  (1)where:
L is the length of the rod after being heated;
L0 is the length of the rod before being heated;
α is the coefficient of thermal expansion for the material from which the rod is composed; and
ΔT is the temperature difference between an elevated temperature to which the rod has been heated and an initial rod temperature (a positive value indicates heating while a negative value indicates cooling).
Area expansion of a two-dimensional plate due to heating is approximated by the equation:A=A0(1+2αΔT)  (2)where:
A is the area of the plate after being heated; and
A0 is the area of the plate before being heated.
Volume expansion of a container due to heating is approximated by the equation:V=V0(1+3αΔT)  (3)where:
V is the volume enclosed by the container after being heated; and
V0 is the volume enclosed by the container before being heated.
The foregoing equations are approximations because the quadratic and cubic terms have been omitted in view of the typical coefficient of thermal expansion α being on the order of parts per million per degree Centigrade. As can be seen, the greater the coefficient of thermal expansion α, the more a work piece will expand when heated and, conversely, contract when cooled, i.e., when ΔT is negative, assuming that α is a positive parameter.
A typical difference in the respective thermal expansion coefficients for the embedded fiber and the resin results in the two components expanding and contracting different amounts when the composite is heated and cooled, respectively. More particularly, resins have a coefficient of thermal expansion that is at least an order of magnitude greater than that of carbon. In addition, resins have a positive coefficient of thermal expansion, while carbon often has a negative coefficient of thermal expansion. Carbon fibers will thus expand when cooled and contract when heated, just the opposite of the resin in which the fibers are embedded. The foregoing factors serve to dramatically increase the thermally induced stress and strain in a carbon fiber composite.
When coupled with the brittleness of the resin caused by cryogenic temperatures, the thermally induced stress and strain cause microcracks in the resin of a carbon fiber composite. Thermal cycling of the composite exacerbates the problem by creating additional cracks with each cycle, and by extending and widening the existing fissures. This eventually causes the structure to leak or to fail when loaded.
It follows that there is a need in the art for a carbon fiber composite structure that will not leak when communicating or storing cryogenic fluids, or fail under cryogenic conditions when under design loads, even after repeated thermal cycling. The present invention fulfills this need in the art.